Healthy soils with high organic content and diverse soil life help plants survive better under climatic challenges such as drought, write the researchers.

They say earthworms can be a convenient indicator of healthy soil life because they are important in soil decomposition.

Rochfort and colleagues investigated whether different farming practices had an impact on earthworms.

'Conventional' and 'biological' farms

The researchers collected worms from farms that were nearby each other and had the same soil properties, but were farmed using different methods.

Two of the farms relied on "conventional" methods using synthetic fertiliser and herbicides. The other two "biological" farms used limited or no synthetic chemicals, relying more on organic fertilisers.

Rather than simply counting the number of worms the researchers used a process called metabolomics to analyse the profile of metabolites extracted from the worms.

Rochfort says metabolomics can be used to show how organisms, which have the same genome, are responding differently to different environments.

Biochemical effect

Statistical analysis of the worm metabolites suggests the environment of the worms had a biochemical affect on their metabolism, Rochfort and colleagues say.

Worms on the "conventional" farms tended to have higher levels of metabolites associated with stress.

Rochfort says previous studies have linked the stress metabolites to copper and pesticide toxicity in soils.

"The study suggests that the worms in the conventionally managed sites may be experiencing greater stress and so may be an indicator of decreased soil health," write Rochfort and colleagues.

They emphasise that further work, including longitudinal studies, is required to validate the results.

Early warning?

Records show the farms studied had been under conventional and biological management for around two years and both had the same levels of organic soil carbon - a conventional indicator of soil health.

Rochfort says the new study suggests worms may provide indicators of declining - or improving - soil health before the farmer notices other, more obvious, symptoms.

"What we'd hopefully be able to do in the long run is advise farmers what the state of their soil health is and help them manage that," says Rochfort.